This disclosure relates to a system for and method of delivering radiotherapy using a x-ray beam for treatment. More particularly, this disclosure relates to a system for and method of determining the location of a particular target region within the body by using imaging techniques that help insure concentrated, high energy radiation is delivered only to the target region, and delivering the radiation so that it is concentrated only on the target region irrespective of patient movement during treatment.
In radiotherapy, for example as practiced in x-ray oncology, it is essential to deliver a precise amount of radiation, or dose, to a precisely defined, predetermined region of a patient's body. Because high levels of high energy radiation are used during radiation therapy treatment it is important that the therapist be able to precisely locate the site to be treated. Before a high-energy treatment machine is used to actually deliver the required radiation for treatment it has been known to use a low-energy imaging machine preliminarily to determine exactly where the dose should be delivered and how it can be achieved. For example, radiation therapists often attempt to use scans from diagnostic CT scanners in planning a radiation therapy treatment. However, in the prior art the relative position of organs within the body during a diagnostic CT scan are not the same as when a patient is placed on a flat couch of the radiation therapy machine. This occurs because the diagnostic CT scanner couch is usually more crescent shaped in cross section than the flat couch of the radiation therapy so that the soft tissue of the patient's body can shift.
Further, standard diagnostic CT scanners tend to be relatively expensive. Therefore, radiation therapy simulators have come into use for initially imaging the target region and its surrounds prior to therapy. A radiation therapy simulator is a diagnostic imaging X-ray machine shaped to simulate the geometry of radiation therapy (or radiotherapy) treatment units, and is typically cheaper than a standard CT scanner. A simulator includes an X-ray imaging source, a gantry to support and position the X-ray imaging source, a couch to support the patient, and an image forming system. The dimensions of the gantry are such that it positions the x-ray imaging source relative to the couch in a geometry mathematically similar to the geometry of the radiotherapy machine. More precisely, the X-ray focal spot for fluoroscopic/radiographic imaging by the simulator is positioned to allow the same target-to-patient isocenter (relative to the X-ray source and imaging detector) as in the radiotherapy machine, even though it is a separate machine. Images formed on the simulator can then be interpreted in terms of the geometry of the radiotherapy machine. Images can be taken from different angles to aid in the planning of how to form and direct the radiotherapy beam to maximize the dose (and exposure time) to the target and minimize damage to healthy organs.
These simulators also have patient couches that are identical to couches of radiation therapy machines.
Beam shaping devices and other accessories can be added to the simulator which attempt to exactly duplicate the therapy setup. Thus, simulators yield a projected planar image of the patient anatomy that is much more geometrically compatible with the position of the radiation therapy system.
In addition to the properly oriented radiographic information, if cross-sectional CT images could be obtained at the same time, then the therapist would be further aided in planning the treatment.
Computed Tomography Simulators
In existing simulators, because the geometry of the simulator attempts to very closely simulate that of the radiotherapy machine, the X-ray imaging source and image forming system are limited to a configuration which is less than optimal for the quality of the image. Both the source and the image-detector-part of the image forming system of the simulator are far from the patient.
An image intensifier has been used to increase the brightness of the image that can be used to produce a television image. A computer has been used to process and enhance the television image.
In the prior art, it is known to form a computed tomography image based on data obtained from a TV camera using an image intensifier tube (IIT) between the patient and a television camera. The output signal from the television camera is processed to form a digital signal that is further processed in a computer to form a tomographic image. This prior art system employing the television camera produces a noisy image of marginal value in simulation and planning.
Similar attempts have been made in the past by various groups to create CT images using X-ray image intensifiers with video cameras. However, from prior CT imaging experience, it is believed that the use of video camera signals based on data off the IIT was one of the major limiting features in these designs. Compared to the IIT, conventional video cameras have horizontal spatial resolution that can produce images with adequate resolution, but their intensity output is both limited and nonlinear. Typically, the instantaneous signal dynamic range of the video camera tube is limited to only two or three orders of magnitude. Conventional solid state video cameras have good linearity, spatially and in intensity, but their signal dynamic range is also limited.
A low cost, computer tomography system designed to be a computer tomography simulator for radiotherapy treatment planning is disclosed in U.S. Pat. No. 5,692,507.
Generally, after the shape and location of target region has been determined with a simulator, but before the patient is actually treated by the high-energy machine, a scheme must be established to deliver the high-energy radiation to the target region. The scheme usually involves determining the dose level and direction of the radiation beam. This includes determining how the beam should be collimated, or shaped, and directed, from different angles such that the predetermined dose will be accurately directed to the predetermined target region. Thus, once the shape and location of the target region is determined from each of multiple angles, the therapeutic beam can be shaped, typically using a multi-leaf collimator, to match the exposed target region from each respective direction of propagation of the beam, and directed at the tumor for treatment. This scheme can be followed at various angles.
The major problem with simulators is that once the target region is determined for the patient with the simulator, the patient is positioned at some later time in a separate radiotherapy machine for subsequent treatment. Often, as much as seven days is required between the planning phase and the treatment phase in order to determine maximum delivery to the target region from the information obtained from the simulator.
Various techniques have been designed to try to ensure that the therapeutic radiotherapy beam is properly shaped and delivered to the target region at each delivery angle. One approach is to place a tight outer garment (such as a corset), with indicia markings, on a patient around the area of the target region. The markings are visible in the images created by the simulators so that the targeted regions can be identified with reference to markings on the garments. An identical garment is worn by the patient during the treatment phase so that the target region can be identified by reference to the markings.
Another approach has been suggested in U.S. Pat. No. 6,104,778, wherein a laser light source is used as a surface marker to help direct the high energy therapeutic radiotherapy beam to the targeted region.
Even the slightest error in positioning the patient relative to the therapeutic high-energy beam of the radiation therapy machine, or movement of the patient while being treated, can displace the target region such that when the therapeutic radiation beam is delivered, it unnecessarily exposes healthy tissue, and partially or completely misses the target region.